1. Field of the Invention
This invention concerns closed thermodynamic devices such as thermosyphons and heat pipes which are often found in many engineering applications such as the direct heating of a working fluid in an Organic Rankine Cycle.
2. Brief Description of the Prior Art
In such devices heat is transferred principally via latent heat evaporation. A fixed volume of heat transfer fluid within a closed system is vaporised by application of heat in an evaporator. Vapour then passes to a condenser where heat is transferred to some other process, the vaporised working fluid condensing against a cooling medium. Once the heat is extracted the condensed working fluid is returned to the evaporator to complete or repeat the process. In most such applications the cycle is continuous and the heat transferred determines the mass flow rate of working fluid being continuously evaporated and condensed. In thermosyphons and heat pipes the significant difference in density between the vapour travelling to the condenser and the condensate returning to the evaporator, is exploited to create a gravity return path, and in such a system the condenser must always be situated at a higher level than the evaporator. However, where the condenser and the evaporator must be at approximately the same level, for example where there is limited headroom, a pump may be used to return the condensate to the evaporator.
In operation of heat transfer devices of the kind described above it is desirable, if not essential, that the closed system contains only one working fluid, or a predefined mixture of fluids, and that no gases are present which do not condense at the working temperature of the condenser.
Of particular practical concern for many such systems is the necessity to exclude air from the cycle which, if present, would tend to collect at the condenser and reduce the efficiency of the heat transfer. Also, such air can affect the pressure/temperature characteristics of the system. In effect, a gas which is non-condensable at the condensing temperature would occupy a volume of the system which is then unavailable for latent heat transfer.
To eliminate non-condensable gases, particularly air, it is common practice to fill or charge such systems by first achieving a vacuum in the empty system before introducing the working fluid as a liquid, taking precautions to make sure air and other non-condensable gases are not introduced. The volume of working fluid introduced into the system in this manner thus defines the available vapour space. This method of charging also implies that such systems may be in a vacuum condition when cold, depending upon the saturation characteristics of the working fluid. Consequently, conditions may allow introduction of air into the system through leakage when the system is not operating. This condition will occur for many high temperature working fluids, including water, ie for working fluid which boils at atmospheric pressure at temperatures above the non-operating temperature of the system.